Quasi-stellar spectra for activation studies in astrophysics

Almost all of the elemental abundances between Fe and the actinides are produced by neutron reactions. The essential mechanisms are the slow (s) neutron capture process, which occurs in the He (and C) burning phases of stellar evolution and the rapid (r) neutron capture process that is characterized by explosive scenarios, presumably supernovae. Typical neutron capture times are years for the s- and milliseconds for the r-process, slow or rapid compared to average γ-decay times. Consequently, the reaction path of the s-process follows the valley of stability, and the so-produced abundances are mostly determined by the effective (n,γ) cross sections averaged over the stellar neutron spectrum. Because of the high density in the stellar interior, neutrons are instantly thermalized, resulting in a Maxwell-Boltzmann distribution. Thermal energies of current s-process scenarios range from kT=8 keV in low mass stars to 90 keV in massive stars. The required Maxwellian averaged (n,γ) cross sections (MACS) can either be determined by folding the energy- differential cross sections obtained in time-of-flight experiments with the stellar spectrum or by activation in a quasi-stellar neutron field. The latter method provides an attractive possibility for such measurements and has been extensively used so far.